ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-11-13029-2011Trends of solar ultraviolet irradiance at Barrow, Alaska, and the effect of measurement uncertainties on trend detectionBernhardG.11Biospherical Instruments, San Diego, California, USA2112201111241302913045This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/11/13029/2011/acp-11-13029-2011.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/11/13029/2011/acp-11-13029-2011.pdf

Spectral ultraviolet (UV) irradiance has been observed near Barrow, Alaska
(71° N, 157° W) between 1991 and 2011 with an SUV-100
spectroradiometer. The instrument was historically part of the US National
Science Foundation's UV Monitoring Network and is now a component of NSF's
Arctic Observing Network. From these measurements, trends in monthly average
irradiance and their uncertainties were calculated. The analysis focuses on
two quantities, the UV Index (which is affected by atmospheric ozone
concentrations) and irradiance at 345 nm (which is virtually insensitive to
ozone). Uncertainties of trend estimates depend on variations in the data due
to (1) natural variability, (2) systematic and random errors of the
measurements, and (3) uncertainties caused by gaps in the time series. Using
radiative transfer model calculations, systematic errors of the measurements
were detected and corrected. Different correction schemes were tested to
quantify the sensitivity of the trend estimates on the treatment of
systematic errors. Depending on the correction method, estimates of decadal
trends changed between 1.5% and 2.9%. Uncertainties in the trend
estimates caused by error sources (2) and (3) were set into relation with the
overall uncertainty of the trend determinations. Results show that these
error sources are only relevant for February, March, and April when natural
variability is low due to high surface albedo. This method of addressing
measurement uncertainties in time series analysis is also applicable to other
geophysical parameters. Trend estimates varied between −14% and
+5% per decade and were significant (95.45% confidence level) only
for the month of October. Depending on the correction method, October trends
varied between −11.4% and −13.7% for irradiance at 345 nm and
between −11.7% and −14.1% for the UV Index. These large trends
are consistent with trends in short-wave (0.3–3.0 μm) solar irradiance
measured with pyranometers at NOAA's Barrow Observatory and can be explained
by a change in snow cover over the observation period: analysis of
pyranometer data indicates that the first day of fall when albedo becomes
larger than 0.6 after snow fall, and remains above 0.6 for the rest of the
winter, has advanced with a statistically significant trend of 13.6 &plusmn; 9.7
days per decade.